My experience with people is that you need to assure that in some form, whether financial, career-wise or whatever, their security lies with you. That is, bind them to you, in every way possible. You need a watertight contract holding them to their word, and the ability to enforce it when the time comes. NDAs are only as good as your ability to pay to have them enforced. Otherwise it's a bluff that will be assumed as being such.
Option #1: Do everything you can, by yourself, until you get investors and thus can pay to have contracts drawn up, enforced etc. Lot of work, but the payoff is peace of mind and zero communications overhead... which itself can take lots of time. You will have to be extremely efficient in your use of time in order not to spend the next 5 years on said project. You could mix in option 3.
Option #2: Find people you genuinely trust, people you have known for a long time. But this can also get tricky when they want to exit, as you don't want to look like you're trying to control their lives, ruining your non-work relations. But you may not have the relevant job titles among your social/family circles.
Option #3: Keep work modular at every stage, so that you never have to let anyone in on the bigger picture. Problem here is having solid engineering and specifications experience to be able to separate the project that way, and ensure things will work once integration time arrives, otherwise you could be severely out of pocket with little to show for it. With this option you can use either volunteers or paid pro's, since compartmentalising the work keeps you safe from blunders.
Option #4: Accept the risks and move forward. You will still need protections in place, however, if only a contract for which the threat of breach is sufficient to deter bad behaviour (Not my ideal choice).